If it is the same as goats, you wouldn't want to do it more than 3 times in 2 years. But that can be hard on a female. Most people only breed once a year. She needs time to recover from birthing and nursing.
From what my Kat breeder says, it all has to do with the individual sheep and her level of condition. Some do very well breeding/lambing 3 times in 2 years and some sheep do best with one annual breeding.
He marks his gals accordingly so he can breed on their schedule. He has one that consistently lambs triplets and can do this on the more frequent schedule. She maintains great conditioning and seems to thrive on this. He has quite a few that breed more often and he likes to save ewe lambs out of that line to increase his flock's overall productivity.
He also "gives them a break" on a schedule also, so as not to burn our his best breeders.
I like to experiment and kind of use my place like my own personal lab, so I had planned on breeding in the spring and fall to see just how the sheep tolerated it. After all, this is why I chose this breed...for their productivity and hardiness.